In U.S. Pat. No. 4,403,337, Kleinman discloses a medical x-ray machine using acoustic pulse-echo ranging in order to automatically determine and set the amount of current and voltage supplied to an x-ray source, as well as the exposure time, based on the thickness of the patient part to be imaged. The ranging system includes a sonic transducer at a known distance from a receptor, and a circuit which determines the travel time of a sonic signal from the transducer to the patient and back to the receptor. The thickness of the patient part can be determined from the actual travel time compared to the time when no patient is present.
In U.S. Pat. No. 4,212,534, Bodlaj describes a device for contact-free measuring of the distance of an object surface from a reference plane having a laser light source producing a beam of light, a light deflector causing the beam to repeatedly scan over the surface of an object and a photodetector. The device measures the interval of time for travel of the beam for at least two specific directions of the beam, and between one of the two directions and a direction at which the detector responds, and uses these intervals to determine the distance.
X-ray or gamma ray apparatus generally produce a divergent beam whose intensity decreases with distance according to an inverse square relationship. Accordingly, in order to deliver a precise dose to a medical patient in order to form a good image on a film plate or to irradiate a tumor without unduly exposing the patient to excess radiation, the precise distance of the patient from the x-ray source must be found. Other factors related to distance which are used in making a precise dose calculation include the thickness of a body part to receive x-rays and the skin surface profile of the area to be irradiated. Previous measurement techniques such as using rulers and calipers to determine distance and thickness, and acoustic measurements, are usually adequate for forming viewable x-ray images on a photographic plate but are not sufficiently accurate for delivering a precise dose to a tumor in a localized area.
It is an object of the present invention to produce an x-ray or gamma ray apparatus with a distance measuring system capable of more accurately determining the precise radiation dose to be delivered to a target area surface of a patient.